This invention relates to extruders, and more particularly to a multi-screw extruder of the type having at least one extruder section formed by a cylinder with a first bore, a second bore, a feed passage, and an extraction passage, by a first screw, and by a second screw, and wherein the bores communicate laterally with one another and have inside surfaces including two common ridges, and the screws accommodated in the bores have mutually meshing threads bounding, together with the inside surfaces of the bores, free spaces containing the material to be extruded, and working zones in which the material to be extruded is subjected to shearing stress.
Multi-screw extruders are used when the structure of the material to be extruded is such that thrust resulting from friction against the inside surface of the cylinder, essential in single-screw extruders, is either insufficient or completely absent. Thus multi-screw extruders are preferably used when the material to be extruded is initially in powder form, especially when the coefficient of friction of the particles of this powder against the inside surface of the cylinder is low, or when the material has a tendency to adhere to the walls. In this case, the multi-screw arrangement, because of the mutual meshing of the threads of the different screws, creates a forced propulsion of the material downstream. However, the main shortcoming of multi-screw extruders is that the material is not homogeneously worked.
German Disclosed Application (DOS) No. 15 04449 describes a multi-screwextruder in which two meshing screw sections have threads of constant pitch but of complementary and gradually varying cross-section. The thread of one of the screw sections increases in width downstream, whereas the thread of the other decreases in width. In this prior art design, the two screws have different functions. The material is advanced only in the free spaces between the cylinder and the thread which increases in width downstream. It is moved while remaining in these spaces and is hence subject to gradual compression during its travel downstream. There is little or no mixing.
In most of the prior art multi-screw extruders, the free spaces associated with the screw threads have parallel functions, but the lack of homogeneity mentioned above also exists. This aspect will be discussed more fully below.
It is an object of this invention to provide an improved multi-screw extruder of the type initially mentioned which ensures homogeneous treatment of the material.
To this end, in the extruder according to the present invention, the feed passage communicates with the first bore and the extraction passage with the second bore, and means are provided for subjecting the material contained in the free spaces of the first screw to a higher pressure than that of the material contained in the free spaces of the second screw.